Control systems for combustion engines are generally designed to reduce exhaust emissions and increase power and fuel economy. The increase in power and fuel economy can be achieved through the use of a suitable spark timing for a given air/fuel ratio. However, any increase in power and fuel economy can be limited by engine knock.
Normal combustion occurs within an internal combustion engine when a gaseous mixture of air and fuel is ignited by a spark plug and burns smoothly from a point of ignition to the walls of the cylinders in the engine. Engine knock occurs when temperature and/or pressure in an unburned air/fuel mixture exceeds a critical level, thereby causing autoignition of the air/fuel mixture. A shockwave is produced as a result that generates a rapid increase in cylinder pressure. An impulse created by the shockwave excites a resonance in the cylinder at a characteristic frequency that is dependent on cylinder bore diameter, combustion chamber temperature, and the like. Pistons, rings, exhaust valves, and other components can be damaged if sustained knocking occurs. Fuel economy and power can also be degraded as a result. Furthermore, persons may find the sound from engine knock to be objectionable. To mitigate engine knock, the spark or ignition timing are adjusted when the engine knock occurs. The ignition timing is adjusted so that ignition occurs prior to or at the moment of autoignition from temperature and/or pressure conditions within the cylinder.
Sensors can be employed to detect engine knock in internal combustion engines. The sensors are used to monitor the combustion process and provide feedback to an engine controller. The sensors are typically mounted on the engine and monitor vibrations transmitted throughout the engine to detect the engine knock. Because the resonance or vibrations caused by engine knock yield a repeatable and identifiable frequency characteristic or spectral signature at a given operating condition, the sensors sense engine knock by identifying the spectral signature.
One type of sensor used for detecting engine knock is a tuned sensor that mechanically or electronically amplifies a magnitude of a signal in the frequency range of the knock-excited resonance. These types of sensors however must be unique to particular engine types due to anticipated frequency ranges of the spectral signature. Another type of sensor is a broadband sensor that has no resonant peaks below the 20-kHz operating range of a typical knock detection system. While broadband sensors can be used in a variety of engine types, postprocessing by a computer or microcontroller is typically required to identify a particular characteristic frequency.
Systems for knock detection typically use a fixed set of one or more filters to perform knock detection on signals received from sensors. Because the filters are usually tuned or adjusted for a particular engine type and range of operating conditions, engine operation outside of this range can result in degraded knock detection or engine failure.